Device discovery is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples of technologies and/or standards, in which device discovery is used, include Bluetooth and several variants of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards suite, such as WiFi Direct. These systems operate in unlicensed spectrum.
Recently, device-to-device (D2D) communications as an underlay to communications in a cellular network have been proposed as a way to take advantage of the proximity of communicating devices, while at the same time allowing devices to operate in a controlled interference environment. Typically, such D2D communications share the same spectrum as the cellular network. For instance, some resources of the cellular network's uplink resources may be reserved for D2D communication. Allocating a dedicated spectrum for D2D communication is a less likely alternative, as spectrum is a scarce resource and dynamic sharing between services utilizing D2D communication and cellular is more flexible and provides higher spectrum efficiency.
Devices that want to discover and/or communicate will typically need to transmit various forms of control signaling. One example of such control signaling is the so-called (discovery) beacon signal, which carries some form of identity and is transmitted by a device that wants to be discoverable by other devices. The other devices can scan for the beacon signal, and once they have detected the beacon, take the appropriate action. For example, a device may try to initiate a connection setup with the device transmitting the beacon. For certain communication modes (e.g., connectionless communication, often employed for groupcast and broadcast transmission), the beacon signal may carry information that indicates associated data transmission to potential receivers. Connectionless communication is typically a unidirectional communication mode that does not require acknowledged connection setup.
The 3GPP feasibility study for Proximity Services (“ProSe”) recommends also supporting D2D operation for out of network coverage user equipments (UEs). In such a case, different synchronization options are possible. For example, UEs may synchronize to a global reference (e.g., a GPS) that is in general different from the synchronization reference of deployed networks. Also, UEs may operate in a fully asynchronous fashion (i.e., no synchronization reference, at least for discovery). A further option is that clusters of UEs synchronize to a specific UE (referred to as Cluster Head (CH)), which provides local synchronization to its neighboring UEs. Different clusters are not necessarily synchronized.
If out of network coverage synchronization is based on sync signals transmitted by Cluster Heads, it is necessary that UEs synchronize to the suitable synchronization reference (CH). A number of procedures may be considered, with some similarities to cell search in cellular networks where idle UEs search for sync signals from different cells and synchronize to, e.g., the cell with best signal strength. Similarly, ProSe-enabled out of network coverage UEs might synchronize to the strongest Channel Head in the proximity.
In some cases, wireless devices reduce power consumption by alternating awake and sleep cycles, which is referred to as discontinuous reception (DRX). During the sleep periods, only the memory and clocks are active and the wireless device is unable to receive any signal. During an awake time, the receiver is on, and the device may be configured to monitor for any control messages. It is often essential that the awake time be set as narrow/infrequent as possible as compared to the sleep time in order limit battery use.